Can't tell you for certain what's available; I know that the
"U.S. Steel Hour" presentation of "No Time For Sergeants"
with Andy Griffith, from 1955, has aired on kinescope on
some PBS stations; likewise, some years ago a CBS executive
in Los Angeles found the kine of the "Playhouse 90" production
of "Requiem For A Heavyweight" with Jack Palance, from 1956.
So there may be more at the networks; I heard several years
ago that there's a large treasure trove of "Studio One" kines in
a Westinghouse warehouse in Pittsburgh.
Your best bet for many of these classic presentations is probably
the film versions; TCM frequently airs the movie versions of "No
Time For Sergeants" and "Requiem For A Heavyweight" (with Anthony
Quinn instead of Jack Palance), and "Marty" often turns up on cable
as well.
Unfortunately, much of NBC's material, including live plays, operas, and
variety shows such as Berle's, Caesar's, and Steve Allen's, were destroyed
years ago; only by the quick thinking of an NBC employee was "You Bet
Your Life" saved and rerun in the '70s.